The Case of the Thingy-Stone
by Maviishroom
Summary: When John shares his carriage with a certain Sherlock Holmes who seems to know more than he should, he presumes that some sort of spell has been cast upon him. Little does he know, things are about to get much, much stranger as he becomes the unwilling accomplice of a 'consulting Auror'.
1. Chapter 1

John Watson stood anxiously upon the platform nine and three quarters, heart still hammering from the decisive lack of impact. Dumbfounded, he stared around at the billowing smoke, flicking robes and most noticeably, disproportionate numbers of hooting, screeching owls. He had presumed that the station would be a come-down after Diagon Alley; however, he had indeed been quite mistaken.

His wonder was brought to an abrupt end as he turned to find Harry watching him smugly. He remembered the bet and groaned.

"You owe me a galleon," Harry declared.

"No, I don't."

"Yes you do, you've got that expression again, you're-"

The two were jerked back to the situation at hand by a word of caution from their mother, a short, educated lady in her mid-forties, and the family wound their way through the crowd, their father pushing the trolley containing the children's luggage. They halted a small distance from the edge of the platform, where they embraced and said their final goodbyes before the two adults helped heave the trunks onto the train.

"And do try to keep out of trouble, Harry!" the woman called.

John felt a brief irritability prickle his stomach; of course, nobody warned him of the same. He had always been the sensible sibling and was neither as audacious nor as inexorable as Harry always had been.

Upon boarding the train, an encompassing anxiety overtook him and he felt compelled to run back and bid his parents a forth goodbye. He resisted the temptation, however, and dragged his trunk through the bustling train, made difficult by his leg, until he reached an empty compartment- or at least, he had thought it empty, until he opened the door to find a small, dark haired boy sitting behind the door, who neither looked up nor gave any inclination he was aware of John's presence.

In truth, John had little fancy for company during long journeys- although well rehearsed in small talk, he found he seldom had enough in common with strangers to occupy any great length of time- but having committed to entering the compartment, manners inclined him to remain.

"Uh, hello," he said, giving a hesitant smile. The boy looked up slowly, first at John's feet then upwards until he met his eyes. "You wouldn't mind-" he let his voice trail off when the other's attention turned towards John's wrist, where his gaze locked. John cleared his throat nervously. He couldn't help feeling that he was awaiting a judgement of some kind.

"First year, muggle born, middle-class family. Sit down, rest your leg. Not that it needs it."

John stared, dumbfounded. "What?"

"You have a psychosomatic limp. Ever seen an explosion? Terrorist attack?"

"We haven't met, I... How do you know me?"

The boy looked back down at the notebook, then spoke rapidly without looking at him. "Oh, I don't. Though I do know that your mother is a doctor and that you have a small dog, probably a terrier. And that your real father died, though not recently, most likely long enough ago for you not to recollect him or at least not with any real clarity. Though you remember how he died, don't you? Do try not to, you're making those memories up."

For a moment, John stared at him, heat rising. Then, "who the hell are you?"

The other's gaze flicked up to meet his momentarily before looking back down at the book. His eyes were rather pale, and John couldn't help thinking there was an odd emptiness about them. Then the situation seemed to click into place.

"You're a wizard. Of course you are, that's how you know this, right? A spell."

No response. He supposed he shouldn't have been so surprised in the first place. "You already know magic, then? You from a magic family, or..."

"Half blood."

"So, how do you...?"

"Legilimens," he said dully. He sounded slightly off, but John was fascinated, if slightly alarmed. He glanced down at his wand, which stuck out of his pocket.

The boy seemed to take note of this. "I wouldn't try it. You won't get anywhere." His voice was of the upmost certainty.

John frowned slightly, struggling to work him out. "I'm sorry, I... what's your name?"

"Sherlock Holmes."

"So, you... familiar with Hogwarts, or..." John quickly registered that the attempt at conversation went nowhere, and muttered irritably, "okay."

"Do you know about the houses?" Holmes asked suddenly.

"No. Or, I've looked at some of our books, so I know of them, but..."

He still didn't look up, obviously only half paying attention. "So which do you want..." he paused, frowning at the book in his hands before continuing, "to be in?"

John shrugged. "I don't know," he said, honestly. "Preferably not Slytherin, judging by the history books."

"Oh, you won't be in Slytherin. No, no." One corner of his mouth quirked up, though the overall expression this gave was impossible to decipher. He unzipped his bag and lifted something out; something smooth whose tongue bobbed out as he watched.

John blinked, surprised, although nothing could shock him anymore. The snake slid through his hands calmly, tongue flicking and emitting a low, fluxuating series of hisses as it coiled around his arm. "I, however," he said, without looking up, "Will be."

A cringing sensation went through John's chest, realising the mistake. "Look, I didn't mean- I-"

But Sherlock no longer seemed to be listening. John watched slightly uncomfortably as he lifted the snake close to his face; the snake seemed too alert for his liking. It emitted a second bout of hisses; he was doubly surprised, however, when the other returned the sound with equal ease.

For some time, he presumed that this was a mere affection, in the same vein as someone would a dog. Yet there was something unnerving about it which he struggled to describe. As hard as he tried to mind his own business, he once caught Sherlock watching him out of the corner of his eyes as he hissed; a second time, he turned his head as John looked over and fell silent, though the snake's dark eyes remained fixed upon him.

The idea that the two were genuinely communicating was one that John first dismissed as madness, and yet, yet...

It wasn't exactly the innateness of the sound Sherlock made, or the subtleties of his expression that spoke of converse. It wasn't even exactly the thought that in this strange new world, it was possible to converse with a snake. It wasn't exactly any of it.

But something about it made his skin crawl.

Just then, the door swung open and a ginger haired boy looked into the compartment, followed by another boy with messy hair and round glasses.

"Sorry, mind if we join you? Someone set off a smoke bomb down there," he gestured vaguely down the train.

"Uh, yeah, sure," said John immediately, moving over to make room. Sherlock said nothing, but fell silent abruptly, scanning the newcomers. The snake seemed somewhat wary; it wrapped more tightly around his arm, partially disappearing into his sleeve.

The ginger-haired boy stared at Sherlock, agape, then quickly sat down next to John. His friend joined him with a curious glance at the snake. He placed a cage on the table between them, containing a large white snowy owl who ruffled her feathers irritably, and dumped an array of sweets onto the seat. John watched the two new pets, remembering the cat he'd wanted to buy, before also remembering that he should probably try to make friends.

"I'm John, by the way," he added belatedly.

"Ron," introduced the red-headed boy, beaming. "And Harry Potter," he said, expectantly with a gesture to his friend. John stared at him blankly.

"He's muggle born," said Sherlock, unexpectedly.

"Oh," said Ron. John got the impression that there was some distrust towards Sherlock there, though they didn't seem familiar with each other. "Never mind."

John blinked, confused. "You're magic, too, then? I mean, not- I mean, you're from a magic family?"

"Yeah. All my brothers have been here." He sounded gloomy. "Harry's family are muggles too, though."

"Oh," he said, trying and failing to think of something more interesting to say.

The last of the journey passed fairly uneventfully. Ron and Harry, who seemed fast friends, chatted amongst themselves; John occasionally joined in the conversation, while Sherlock sat in silence, seeming to have bored of their company.

By the end of the journey, John held several chocolate frog cards, gifted to him by Harry, who seemed equally as fascinated as he was, and had watched Ron try several times to turn his rat yellow, an apparently impossible feat.

As train rolled to a standstill John gathered up his belongings, checking around him to make sure he left nothing behind. A small piece of glass glinted at him from under Sherlock's side of the table. He reached down and held it over his hand. It had two strips of plastic either side of the glass, and magnified the pattern of his skin hugely. He frowned, turning to look for Sherlock; however, he had disappeared the moment the train slowed, and was nowhere to be seen.

Then he caught sight of the castle towering above them, and the glass slipped into his pocket, forgotten.

John couldn't remember feeling this nervous since Harry had told him she'd convinced their headmaster he had stolen the door of the girl's bathroom.

He watched as a Moriarty, James was sorted into Ravenclaw and walked off, giggling oddly. Sherlock, after a long pause, was indeed sorted into Slytherin.

John watched Holmes go with a slither of gratitude and a bout of relief that somewhat calmed his nerves. At least that was one person he wouldn't have to share his house with. Supposing Sherlock was correct, in any case. His adverted his eyes from the crowd modestly, shuffling his feet. He couldn't help feeling a slight discomfort towards the Slytherin table; the upturned faces all seemed to have the somewhat bored arrogance Sherlock's had during the train journey.

He supposed that it was his good fortune that he knew virtually nothing about the houses; at least this way he lacked the added anxiety of worrying which house he would be allocated to.

He was one of the last to be called, and stumbled forward somewhat ungracefully before jabbing the hat onto his head. He flushed, realising he must have looked a fool. He consoled himself with the thought of the boy who had forgotten to remove the hat upon leaving the stage.

The hat promptly shouted, "Gryffindor," and John took his place at the red and gold table to thunderous applause; luckily so, as he had been thoroughly confused as to which table was which. He was joined by his sister, then Ron, and the sorting ceremony ended, promptly followed by the largest school dinner John had ever come across.

"First years, hurry yourselves up," called a dark haired boy, whom John judged to be somewhere around his fifth year. "Some of us want to sleep."

"Who's he?" John asked, to nobody in particular.

"Greg," answered a ginger haired boy, who seemed to be Ron's brother. He didn't bother to lower his voice. "He's Quidditch Captain now, but the hat tried to put him in Hufflepuff."

"Oi, watch your mouth," he replied evenly.

Before long, Greg's wish was granted and people started to filter out of the hall; another ginger haired boy led the way out (John wondered exactly how many brothers Ron had), and the group squeezed through the door in unison with Slytherin. Catching sight of Sherlock, John chased after him, catching up just outside a dim looking corridor.

"Hey," he panted.

Sherlock held out his hand silently.

"Uh," said John. Frowning, he shoved his hand into his pocket and handed the glass piece to Sherlock. He noticed slightly uncomfortably that an older boy had stopped and was watching them, eyebrows slightly raised. "Well, I better, er-"

The older boy's lips upturned tightly. "Better what? Catch up with them?"

John spun around, staring at the empty hall.

"Come, Sherlock."

"Wait, I-" John started, but they disappeared around the corner and were gone.

"Great," he muttered. "Just... great."

He jogged slowly in the direction they had been going, limping and cursing Harry for forgetting him. He reached a long winding staircase, which he presumed they must have followed. He came to a dead end and turned back; he found a side alley and entered a long corridor, with far too many offshoots and dead ends to navigate; and eventually came to a kind of staircase bridge between two walls, pocketed with various entrances and openings. He flopped down, defeated. He was more than a little annoyed at being left behind, and even more so at not being missed.

"Never did catch up, then."

John looked up to be met by Sherlock's unapologetic stare. He let out a huff of disbelief. "You bloody-"

"Yes, well, it was a 'shame' to stop there," he said, an odd enthusiasm in his voice.

He stared at him blankly.

"It's just around the corner," Sherlock said, humour making his voice quaver. "It's just around the corner, and you-"

John stood abruptly. The sudden change made stars burst in his head, but he gave no outward sign of it. "I'm glad you," he snapped, "find it so funny." He spun around, not really caring why the Slytherin was out of bed so late, and not really caring how their paths had ended up crossing in such a maze of corridors. He turned the corner. There were no lights on; but he supposed it was night-time, so that was a given. He reached the door and yanked on the handle aggressively, but it was locked. "Oh, great," he muttered, for the second time that night.

"Here," said Sherlock, appearing noiselessly at his side. "Alohomora."

The door unlocked with a satisfying click. John put his hand on the handle, but didn't turn it. He looked at Sherlock, lips pursed, opened his mouth to make some kind of angry retort, then closed it again. He opened the door and stepped inside, slamming it on Sherlock, who immediately opened it again, some kind of manic excitement in his eyes.

Which John did not share.

At all.

Because at his feet, there lay three giant mouths. Towering above there hovered the three giant heads to which the mouths belonged. And there lay, beyond them, one giant body to which all three belonged.


	2. Chapter 2

John leant against a large and musty wall, gasping for breath and adrenaline making his heart pound.

He couldn't say it was a bad feeling.

He grasped at his robes, where a section had been torn. Come to think of it, he didn't even recollect it being torn; all he could think of was the snarling faces of the... dog? Monster? He mentally decided on monster. "So... was that... supposed to be in the common room, or-?" He hadn't realized he had spoken before he did so, and laughed shortly in a voice which seemed disconnected from his brain.

"I thought Gyffindor's emblem was a lion."

He let out a slightly hysterical giggle. "Jesus, was there a giant snake in yours?"

John wasn't sure if he should be joking or not, but he couldn't help bursting into laughter at the look Sherlock gave him. It was a long time until either of them could stop.

"That thing ripped my robes," he panted eventually, voice oddly high with mirth.

Sherlock snorted. "Would have ripped your legs off, too."

"Did you pull me back or did I fall?"

He seemed to consider this for a second. "Both."

John huffed and flopped back against the wall. "I am never taking direction advice from you..." he said, slipping down into a sitting position, "ever again."

"I should think not, Mr Watson. Explain yourselves."

He jumped and looked simultaneously in the direction of the noise and straight up into the face of the most irritated woman he'd ever seen in his entire life. Any remaining laughter dried up immediately.

He stammered the beginnings of several sentences to which no endings suggested themselves before managing, "I got a bit- left behind," in a strained voice.

She raised her eyebrows. John winced slightly in recognition; Professor McGonagall was his new head of house. He shuffled his feet awkwardly.

"I'm aware. Nobody informed me of your friend here being out of bed, however."

"He had to give me something back I'd left on the train," Sherlock said, suddenly sounding very honest. A little too honest.

The professor's lips thinned. "Yes. That doesn't explain why you are out of bed."

Sherlock's eyes widened slightly. "I went back because I realised John was probably lost," he glanced down at the floor. "I thought I knew where to go, but we got lost and- well, we ended up here." He looked up again in an elaborate facade of innocence. "We're not in trouble already, professor?"

It occurred to John suddenly that he never did ask exactly what Sherlock was doing, wandering in the dark on their first night.

"Hm." Something gave John the impression McGonagall recognised the lie, but didn't call them out on it. "Not this once, though I doubt your brother will be pleased." Her lips grew even thinner. "And I must inform you that this is the third corridor of the fourth floor, off bounds as you were informed by Professor Dumbledore, and that should I find you here again, you will both face serious repercussions."

Sherlock nodded solemnly.

Then, John couldn't help himself. "So, we going to bed, or..." he trailed off at the look McGonagall gave him. '...should I just shut up,' he finished internally.

He had a feeling McGonagall gave him what was almost a smile. Or at least, it wasn't a death stare. "Follow me."

John and Sherlock followed her down a series of corridors until they reached a portrait of a rather pudgy looking lady asleep in a frame. John's curiosity had been rather spiked by Sherlock's lie, and it was with some regret that he climbed through the portrait hole, helpless to question him in front of McGonagall.

He stumbled up the staircase and collapsed onto a spare bed, hoping he'd found the right dorm. He didn't really care.

John woke abruptly the next morning, surprisingly lacking his usual tiredness, and certainly not feeling any of the expected effects of four hours of sleep. He slipped out of bed and brushed off his robes, thinking that he should probably change them, then that he couldn't be bothered to.

Nobody else seemed even vaguely awake (he was pleased to see that he had in fact got the right dorm). His legs felt restless and eager to explore; carefully, he opened the door and wandered down into the common room.

"John!" an ecstatic voice exclaimed.

John groaned. "Harry," he said, bracing himself for an oncoming tirade of questions.

"Where did you go?"

"Uh-" for a second, he considered that McGonagall probably wouldn't appreciate him spreading the fact that there was a giant three headed dog on the third floor, but somehow the fact seemed to spill out within close to three seconds.

Harry oggled at him. "What?!" She looked thunderstruck. "That's flipping amazing! Third floor? We're going to look." She grabbed his robes fixedly.

John winced, shaking his head rapidly. "No, no, no, McGonagall caught us, she'll murder me if I go up there again."

"Us?"

"Uh, yeah." He didn't much feel like relaying the whole event, but soon yielded; partially because he knew he had no hope of not doing so, but mostly to see the jealous look on Harry's face for once, instead of having to wear it himself.

By the time he'd finished, she looked both thrilled and predictably envious. "What, so- d'you reckon he did it on purpose?"

John frowned, abashed by the unexpected question. "No, why would he?"

"Dunno," she shrugged. "Epic prank?"

'Just because that'd be your idea of a laugh,' John replied mentally. Then, "Nobody knows it's there."

Harry paused, then shrugged again. "Huh, I guess. Wish I'd have been there. Wait till I tell mom ickle baby John got in trouble on his first day, she'll-"

John started. "You're not telling mom!"

"Watch me!"

"I'll- I'll curse you!"

Harry gave him a condescending look. "No. No, you won't."

He privately agreed, but wasn't about to admit it. He pulled his wand out and waved it threateningly. His wand didn't comply, refusing to send any sparks flying; he shoved it back into his bag while Harry laughed raucously.

The week passed without further event. John didn't see Sherlock again; not even in passing, bar glimpses at mealtimes. He always seemed to leave early, either alone or accompanied by an older Slytherin, whom John supposed was his brother.

It frustrated him some, still curious as to what he had been up to that night, but he supposed he'd get a chance to ask sooner or later.

As it turned out, that opportunity came that Friday.

"Urgh," said Ron through his glass of breakfast juice.

John glanced up. "What?"

"Double potions with Slytherin."

"Oh," said John, who was both eager to experience a potions class and to be in the same room as Sherlock. "Problem?"

It seemed he was one of the few people who felt as such, however. He trailed out of the hall behind Dean and Seamus, perplexed. While the Slytherin table didn't seem as welcoming as the others, he hadn't noted any real reason to dislike them otherwise; after all, Sherlock did save him from getting his leg lopped off, despite having caused the incident.

The dungeons, where potions was held, turned out to be both cold and rather dimly lit. John's heart sank when they were told to separate into twos and threes. It wasn't that he hadn't made friends, exactly; more that he didn't spew out his life story the moment he met anyone, and therefore had less to say than the rest of them.

Still, it didn't make watching everyone separate into groups while he stood lamely in a corner any easier. He limped to a spare table at the back of the class, not wanting to have to ask to join someone. He threw his bag on the table and tried to look busy.

"Mind if I join you?"

Sherlock dropped his bag onto the table before John had a chance to reply. John couldn't help grinning, though Sherlock didn't return the smile. As they unpacked their books, a small slip of paper caught John's eye as it fluttered down onto the floor. He frowned, reaching down.

Sherlock's eyes flickered over him, and he scooped it back up and stuffed it into his bag before John could get a closer look. But not before John had read the words 'BREAK IN' in the bold headline.

"What's that?" he said, looking from Sherlock to the bag, where the slip had been concealed.

He glanced at him, nonchalant. "It's a newspaper clipping."

John frowned. "Yes, I know."

Sherlock watched him for a minute, but didn't give any response to the comment. Instead, "No luck with friends, then."

"Excuse me, what?" John said blankly.

He narrowed his eyes very slightly. "You didn't even try to stand with them, why don't they like you?"

John blinked. "They do like me. They just..." he trailed off, "they..."

"Like each other more."

That stung a little. Mostly because it was true. "Yes," he snapped. "Thank you. You know, you don't seem to have bunches of friends either, so-" he wanted to say 'shut up', but didn't quite summon the rudeness to do it.

Sherlock cocked his head to one side, with an expression John couldn't quite define. If anything, he looked interested. John didn't really appreciate being looked at like a mildly interesting specimen. He busied himself in scanning the instruction list for the potion they were supposed to be making, no longer feeling very conversational.

"Ask it," said Sherlock suddenly.

Surprised, John looked up from attempting dice an odd looking plant. "What?"

"If I think the object someone attempted to rob from Gringotts is through the trapdoor, too."

John stared at him blankly. "I have no idea what you're going on about."

He pulled a face, frowning. "You must," he said. There was a quizzical look in his eyes. "You were there, and you've been trying to ask me something all week."

Baffled, John was quiet for a moment. Then, "I was going to ask why you were out of bed. How-"

"Disappointing," Sherlock interrupted, seeming to lose interest and persist with his potion.

"No, go on. What about a trapdoor?"

He didn't look up. "You were there."

"I never saw a trapdoor."

"The dog was standing right on it." When John didn't reply, he dropped his pen onto the desk with a clatter. "I don't understand, how are people so blind?!"

John raised his brow slightly. He didn't quite understand what he was getting at, and was more than slightly inclined to think he wasn't quite sane. "So... what got robbed from where, and why did you..." he sighed and shook his head, thinking better of asking.

Sherlock watched John for a few moments, while John desperately pretended not to notice. Then, in quick succession, "Somebody 'tried' to rob something from a high security vault at Gringotts- actually got in and out again, notoriously hard to do- but the object was removed the day before. Supposedly nowhere safer than Gringotts, so why did they remove it? Where else could something so desirable be stored, bar Ministry facilities?"

He said this all so fast that it took John a moment to register it. "Uh, I dunno," he said, stupidly. He could feel the intrigue building in his stomach.

Sherlock didn't seem to have listened, anyway. "Then Dumbledore announces a corridor on the third floor is out of bounds with no good reason why," he finished.

John laughed shortly, incredulous, and shook his head again. "How could you possibly connect those two?"

"I didn't. I'm not. But we know it's guarding something. Would be the point of having the dog there standing on a trapdoor if it wasn't guarding something valuable?"

"Doesn't mean it's the thing from the news."

It was a fanciful idea of someone who was showing off; they both must know that. Either that or Sherlock was some sort of fantasist. After all, John hadn't even seen a trap door, and this was a magic school. But he couldn't help it; part of him wanted Sherlock's crazy theory to be true.

John hesitated, and then something clicked into place. "Wait, so you actually did send me in there on purpose?"

"Only so you wouldn't tell Professor McGonagall I was trying to break in there when she arrived. I didn't expect alohomora would open it."

"When she arrived? When, not if?"

Sherlock rolled his eyes. "Obviously. Urgh," he added, seemingly to himself; he scowled at the potion instructions. "Snake fangs." He hissed shortly as he added the ingredient, caressing his bag, where John supposed he must have his snake.

He bit his lip. He thought he should probably not question it, considering how odd he'd been about everything else, but couldn't stop the words tumbling out. "Why do you keep doing that? Hissing like that?"

Looking up, Sherlock met his eyes for a split second, then looked away and said very quickly. "You're doing it wrong, take it off the heat first."

"Ah," John said, cringing at the potion which was now making him pay for not giving it the attention it deserved; he had turned an odd, sticky blue colour. "Right, thanks."

"You have defence against the dark arts. Then the afternoon off."

It took John a moment to register that he was, in fact, correct. "Uh, yeah," he said slowly. "How did you know?"  
Sherlock gave him a smug look. "Why else would you have put your defence against the dark arts book in your bag," he replied. It should have been a question, but it wasn't.

John honestly couldn't tell whether he had just seen his books or whether he was using whatever magic spell it was that he had used on the train again. He nodded, grinning slightly. "Right. Ok."

"It's a bit dull, isn't it?" Sherlock said flatly.

"Defence against the dark arts?" He made a non-committal noise. "It's interesting enough."

Sherlock fixed him with a pale stare. "Fancy skipping it?"

"God, yes."

 **Hi! Thanks for reading, if you've read this far. This is my first fanfic, so I'm not sure that it's my best work, but I hope you're enjoying it! Any feedback is much appreciated, and I do live for comments. That said, as long as someone's having fun with it, I'll be happy!**


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